U.S. Pat. No. 4,651,513 describes a steel cord for reinforcing rubber products comprising two or more successive wire layers of which an inward layer comprises wires coated a corrosion resistant coating and the outer surface layer comprises wires with a rubber adherable coating such as brass. The referenced corrosion resistant coatings in this application are zinc or a zinc binary or ternary alloy that comprises at least 50 wt % zinc. These coatings are alternatives i.e. the application does not mention that the coating can consist of zinc together with a zinc-alloy on the same wire.
EP-B1-1280958 discloses a steel cord adapted for the reinforcement of thermoplastic elastomers. The steel cord is a multi-strand steel cord. At least some the steel filaments have a zinc-iron alloy layer and on top of this zinc-iron alloy layer a separate layer of mainly zinc. The thickness of the separate top layer of zinc—not including the alloy layer—is smaller than two micrometer. This intermediate layer of a zinc-iron alloy and a relatively thin top layer of a zinc layer are obtained by means of a hot dip operation. The steel filaments are dipped into a bath of molten zinc. Instead of leaving the bath vertically, the filaments leave the bath under a small angle with respect to a horizontal line and a great amount of zinc is wiped off mechanically.
As mentioned in EP-B1-1280958, the resulting steel cords with such steel filaments have several advantages. First of all, due to the thin zinc layer, there are only a small number of separate zinc particles and less zinc dust is created in the processing of the steel cords. The reduction of zinc particles and of zinc dust increases the adhesion level. Secondly, due to the zinc-steel alloy layer, corrosion resistance is still much better than in the case of steel filaments which have been coated with zinc by means of an electrolytic deposition method. Thirdly, due to the zinc layer and zinc-iron alloy layer being also thinner, the level of fatigue resistance has significantly increased. Steel cords according to EP-B1-1280958 have given satisfactory results not only on a lab scale but on a wide scale in various industrial applications.
This wide commercial use, however, has also highlighted some points which are open for improvement.
First of all, although very thin, there is still zinc at the surface and zinc is known as difficult to twist in a downstream operation. Either the speed of twisting is seriously reduced, or lubrication becomes unavoidable. After the twisting process, the added lubricants need to be removed, since the presence of these lubricants would be at the detriment of the adhesion level in a polymer or elastomer matrix. Experience has shown, however, that complete removal of the lubricants is costly and time consuming.
Secondly, the presence of zinc at the surface, may lead to processability problems at the customer. An example is the extrusion of polymer strips around steel cords, if the steel cords have to pass through small openings before entering an extrusion apparatus, the steel cords rub against the wall of the openings; zinc becomes loose, heaps up locally and eventually blocks the whole processing. As will be described hereunder, the strips may show dark spots indicating the presence of zinc dust or may even loose their planar character. In extreme cases the steel cords were broken due to zinc dust blocking the extrusion dies.